EATING more oily fish can increase brain size in later life and may help prevent age-related mental decline, a study has found.

People with higher levels of omega-3 fatty acids found in fish oil preserve bigger brains as they age, the research shows. In particular, they maintain more nerve cells in the hippocampus, the brain's key memory centre.

The larger neural volume linked to omega-3 intake is equivalent to reducing the effects of brain ageing by up to two years, say scientists.

Researchers measured levels of two essential omega-3 fatty acids, EPA and DHA, in the blood of 1,111 women taking part in the US Women's Health Initiative Memory Study.

Eight years later, when the women had an average age of 78, magnetic resonance imaging (MRI) scans were performed to measure their brain size.

Those who started out with higher intakes of omega-3 were found to have larger total brain volumes. A doubling of omega-3 levels was associated with a 0.7% increase in overall brain size.

In addition, the hippocampus area was 2.7% larger in women with higher levels of the fatty acids.

Lead scientist Dr James Pottala, from the University of South Dakota in Sioux Falls, US, said: "These higher levels of fatty acids can be achieved through diet and the use of supplements, and the results suggest that the effect on brain volume is the equivalent of delaying the normal loss of brain cells that comes with ageing by one to two years."

The findings are published in the latest online edition of the journal Neurology.

Shrinking brain volume is a known sign of Alzheimer's disease as well as normal ageing.

Dr Laura Phipps, from the charity Alzheimer's Research UK, said: " There has been mixed evidence as to the benefits of omega-3 fish oils on the brain and whether they may protect against memory decline and dementia.

"This study suggests that higher levels of omega-3 fatty acids in blood are linked to larger brain size but the possible reasons for this association need further investigation. We know that the brain gets smaller in people with dementia, but it is unclear from the study what effect larger brain size would have on memory and thinking in the volunteers or their long-term risk of developing dementia.

"The best way to assess whether omega-3 could protect against dementia is through clinical trials and so far, trials of omega-3 supplementation have not shown benefits in protecting against cognitive decline.

"Currently, research suggests that a balanced diet, along with regular exercise, not smoking, maintaining a healthy weight and keeping blood pressure and cholesterol in check may help to keep our brains healthy as we get older."

Dr Doug Brown, director of research at the Alzheimer's Society, said: " It's interesting to see that eating more fish could lead to larger brain volume, particularly in the hippocampus - an area of the brain that comes under attack in dementia.

"We know that brain shrinkage can be linked to dementia and larger brain volumes could indicate a better ability to cope with the ravages of the condition, but it's a big leap to draw this conclusion.

"Whilst interesting, this study still leaves us in the dark about what effect eating fish has on the development of dementia."